This invention relates to a heavy-duty pneumatic radial tire used for vehicles, such as trucks and buses, and more particularly to a heavy-duty pneumatic radial tire improved with respect to the prevention of a wandering phenomenon.
In recent years, radial tires are used for heavy-duty vehicles, such as trucks and buses, as well. They exhibit unique excellent high speed performance, wear resistance and low fuel consumption. In order to withstand a heavy load, heavy-duty radial tires are provided on their tread portions with a reinforcing belt, thereby remarkably increasing the stiffness of the tread portion. Further, they are used in such a state that they are filled with air so as to give a high internal pneumatic pressure.
However, when such a heavy-duty radial tire, provided with a tread portion having high stiffness and filled with air so as to give high internal pneumatic pressure, travels on a road having a "rut", it receives excessive external force from the road surface, where it gets out of the rut. This makes it impossible to control the steering wheel, i.e., causes the so-called "wandering phenomenon." The wandering phenomenon is characteristic of heavy-duty radial tires, and is not caused with bias tires and radial tires for relatively light-duty vehicles such as passenger cars.
Since camber thrust is a major causative factor of the wandering phenomenon, tires having rounded shoulders were proposed in order to prevent this phenomenon. However, no fully satisfactory function of preventing the wandering phenomenon could be attained merely by rounding the shoulders of tires, though some improvement was recognized when compared to conventional tires having square shoulders.
For this reason, there have been proposed a tire having not only rounded shoulders but also narrow zigzag grooves provided on the inside of each shoulder along the circumferential direction of the tire, as disclosed in U.S. Pat. No. 4,214,618 (corresponding to Japanese patent publication No. 56-40044), and a tire having lateral ribs which have been formed on rounded shoulders by partition with narrow grooves so that they have a height lower than that of the ribs provided on the inside of the lateral ribs, thereby proving a difference in level therebetween, as disclosed in U.S. Pat. No. 4,480,671 (corresponding to Japanese patent application Kokai Publication No. 58-194606).
In the former tire, the narrow grooves have a zigzag shape. Therefore, the width of the lateral ribs periodically varies along the circumferential direction of the tire. In other words, the stiffness of the lateral ribs varies along the circumferential direction of the tire. This causes in an early stage an uneven wear such that the shoulder portions are worn into polygonal shapes, which deteriorates the cornering performance of the tire. On the other hand, the latter tire has a difference in level between the lateral ribs and the ribs provided on the inside thereof. The difference in level tends to accelerate the wear of the shoulder portions, leading to uneven wear.
Studies conducted by the present inventors revealed that the wandering preventing performance of a heavy-duty radial tire can be further improved by providing the narrow grooves near the edges of the shoulder portions as much as possible and, at the same time, narrowing the width of the lateral ribs. In the above-mentioned conventional tires, the wandering preventing performance was not always satisfactory, since the narrow grooves are arranged at a relatively long distance from the edges of the shoulder portions.
On the other hand, when the narrow grooves are provided near the edges of the shoulder portions as much as possible, the wandering preventing performance can be improved. However, this method lowers the stiffness of the lateral ribs, which raises another problem that cracks tend to occur at the bottom of the narrow grooves. If the narrow grooves are provided too near the center of the tread in order to eliminate this disadvantage, not only the wandering preventing performance is spoiled but also the wear resistance is lowered due to the decrease in the tread development width. As is apparent from the foregoing description, mere change in the position of the narrow grooves does not lead to satisfactory solution to the problems accompanying the improvement in the wandering preventing performance.